These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Big Lipped Alligator Moment: There's a moment in Trigun Maximum where Vash and Livio are counting the coins they got from the Gung-ho guns. They were missing one. Suddenly, a muscular black guy with weird hair and skimpy disco attire comes in and gives them the coin, pulls a bug out of his mouth, then proceeds to ask whether Vash or Livio prefer clams or fish. Granted, there's a reason why this happened (Zazie used her mind control worms), but it's random, it's weird, and after the incident, nobody talks about it again.
Creepy Awesome: Legato, who is a fan favourite because of his disturbing mein, creepy voice, and love of tormenting Vash.
Draco in Leather Pants: Almost too many to count. Legato makes fangirls pass out all over the place (even though he's gay, and was even a gigolo — only in the manga, though). Knives has a fervent fandom too. Which is especially creepy given it's implied that Legato is a cannibal.
Also, Livio in the manga, goes from a wild card to The Lancer after Nicholas' death.
Evil Is Sexy: Legato, Knives, and a few of the more minor Gung-Ho Guns may count. Legato in particular is a pretty extreme case considering he's probably the evilest character in the story...and also probably one of if not the best-looking (which really says a lot). Manga Knives also has more outfits than anyone else.
Fan Wank: Not as bad as in other fandoms, but still pretty bad at times.
Foe Yay: Particularly egregious when Legato seems to be hitting on Vash. It's made stronger by the fact that Legato genuinely is gay and he also is fixated upon Knives, who is Vash's identical twin.
Freud Was Right: Knives in the manga. Possibly Vash's view of Rem depending on how you see it.
"Funny Aneurysm" Moment: When someone says that the cross Wolfwood carries on his back is heavy he replies "That's because it's full of mercy!" Later on, that line becomes a bit sinister.
Germans Love David Hasselhoff: Trigun is ridiculously popular in America, but not quite as much in its homeland of Japan. Because of this, the movie premiered at Sakuracon 2010, months before the Japanese premiere.
The anime starts out as goofy, almost slice of life style misadventures as Vash roams the desert getting into trouble while Merril and Milly desperately chase after. Then it turns into a darker, grittier story as Vash's Evil Twin rears his sinister head and Vash in caught up in a dark and horrible story of Sibling Rivalry that examines just how much a person can really expect to uphold the ideal of Thou Shalt Not Kill.
For Vash specifically, this might be better termed Showing the Beard, as he's actually quite competent from the very start, he's just a CloudCuckoolander/prime example of Obfuscating Stupidity most of the time so it's not terribly obvious. Opinions vary as to exactly where this happens in the anime, but it's pretty early; brief flashes of Vash's badassery appear as early as the end of Episode 2, and very few would put it later than Episode 5.
Ho Yay: Vash and Wolfwood have a lot of romantic-ish interactions with each other. Legato is obviously in love with Knives and tells Vash he's "beautiful" in the unedited version of the manga.
Iron Woobie: Vash. Seriously, nothing is allowed to go right in his life, and he loses everything and takes it very much on his own head whenever anyone gets hurt on a half-civilized planet with almost no natural resources and loads of guns. And it took him over a hundred years to briefly give up.
It Was His Sled: Most of the anime's big twists have long since passed into common knowledge in the English-speaking anime community by now, to the extent that when [adult swim] chose one episode to air for their 2012 April Fool's Day Toonami block, it was the one where Wolfwood dies.
And Gasback in Badlands Rumble - he pulled three major heists after the Time Skip, which at first looks like a simple matter of revenge against his former accomplices. First was a casino with a giant inflatable mascot, next was a cargo hovercraft plant. The final one, though, revealed that the first two also led up to the final heist, which was of the entire plant for a town run by his final accomplice, using the casino's mascot as a cushion to keep the plant's bulb unbroken, and the hovercraft to move it after it'd been led downhill.
Moe: Let's face it, Vash is a very Moe character. You just have to see the visceral Squees he seems to trigger all over the fandom.
Narm Vash and Knives's whining pathetic ways of arguing about their very extreme ideologies bring out their emotional immaturity, as they can't seem to get more nuanced than "everybody is good vs. everybody is evil."
Some Anvils Need to Be Dropped: Despite everything else about the series, the underlying message of the series is that killing people is wrong, and almost never justifiable.
Squick: In the manga, when Zazie the Beast pulls a worm out of her crotch.
Some parents consider this to be an anime for teens ages 15-16 when it's more specifically created for kids around their early teens. Some people do consider this anime for kids, but since it's aimed at kids a little younger compared to Cowboy Bebop, it could also be for a preteenage group.